The present invention relates to a method of, and apparatus for, the calibration of a machine system in which relative movement takes place between two parts of the machine. Examples of such machine systems are co-ordinate measuring machines and machine tools.
Such machines generally have parts movable along three mutually orthogonal axes generally designated x,y and z axes, by means of which tools, which term includes measuring devices such as touch trigger probes, are positioned with respect to a workpiece. The machines include measuring devices for example, scales, positioned along the three axes of the machine, and scale readers which provide a measure of the movement of the movable part of the machine, thus providing a determination of the position of the tool along all three axes. The scales themselves are calibrated during manufacture.
The movable machine parts run in tracks on the fixed parts and errors arise between the actual movement of the movable parts, and the movement indicated by reading the scales. These errors are due, inter alia, to pitch, roll and yaw movements of the machine parts caused for example, by mis-alignments or tolerances in the tracks. There may also be mis-alignment of the scale on its axis or between the scale and scale reader.
It is known from U.S. Pat. No. 3,654,446 to account for machine errors of the sort outlined above, by plotting an error map of the machine. This is achieved according to the patent specification, by moving the movable parts of the machine to a large number of positions within the working envelope of the machine and, at each position once the machine has stopped, recording the counts of the machine scale readers. The actual distances moved by the machine parts along the three orthogonal axes are then independently measured by another measuring device in the form of a laser interferometer. The laser interferometer includes a counter for counting interference fringes produced within the interferometer during movement of the movable parts of the machine. The difference between the measured movements of the movable part of the machine as indicated by the scale reader and the interferometer fringe counter along each axis, are then recorded in analogue or digital form on any suitable recording medium.
While this method can provide useful information for correcting many of the above-mentioned errors, workpieces are usually measured while the machine is in motion, and the above-described method will not take account of any other errors which may be introduced due to movement of the machine.